How much tank pressure do you usually lose during initial forced carbonation?

I hooked up my gas line to the keg set to deliver 12 psi (set & forget). In the past 4 days my tank pressure has gone from around 900 psi down to around 400 psi. Is this normal, or should I step up my searching for leaks (none obvious).

Pressure doesn't drop as CO2 is used. The only thing that can affect tank pressure is temperature. You probably have the tank in your fridge right?

If you had 5 gallons of brew and you're carbing to say 2 volumes, you've used exactly 10 gallons of CO2. I have no idea what that translates to in pounds of liquid CO2 (which is how you'd measure how full a tank is), but it's not much.

If the pressure drops but the tank temp has remained the same, it means you have no more liquid CO2 in the tank and you're basically empty. However, when CO2 is used quickly, like when applying to an uncarbed corny for the first time, the temp will drop and show a lower pressure. Put the tank on a scale and then subtract the tare weight which is usally stamped into the tank. That's how much liquid CO2 you have left. A 5lb tank would have about 5lbs in it when full and about zero when empty.

I got a used keggerator off craigslist & the guy threw in the 5 lb co2 tank, it sits outside the unit. It had about 900 psi when I first put the regulator on it.

I've heard that when these tanks start to get low, they run out quick. I was thinking low was below 500, I thought I'd still be good with 900

On my way to the gas store now (right down the street, score!). I'll use your star san technique tonight.

Thanks!

Sounds like you tank was close to empty to start with is just about empty now. So that's a good call just to go ahead and get it filled. It never hurts to check for leaks though just to be sure. And a tank will empty pretty fast if you have a leak.